Postgresql – Postgres JSON field for many to many relationships

I'm designing a schema that has a many-to-many relationship as such:

A lead can have several stages and each stage can have several leads. I currently have three tables, with one of them as being a junction table.

stages
----------
id | name | workflow_id | created_at | updated_at

leads
----------
id | name | created_at | updated_at

stage_leads
--------------
id | stage_id | lead_id | created_at | updated_at

(stages belong to a workflow)

I've got indices set up.

**

When a lead moves from one stage to another, a new entry is created in the stage_leads table. My use cases seem simple:

a) find/count all leads on a particular stage, and

b) find/count all stages a particular lead has gone through

My concern is that since every move is creating a new entry into the junction table — and there are potentially hundreds of stages a lead can go through — it may lead to hundreds of thousands of rows for very few leads and that could probably impact the lookup time when I'm trying to fetch a specific set.

My question is this: would it be better to use Postgres' JSON functionality to store my data something like this:

stages
----------
id | name | workflow_id | created_at | updated_at

leads
----------
id | name | current_stage_id | stages | created_at | updated_at

… where the stages column on "leads" is a JSON object that logs the stage_ids that the lead has gone through, the timestamps, and a couple of other fields?

This seems to satisfy my use case of finding all leads on a particular stage (I can run a query against the current_stage_id of all leads) and it probably makes retrieving the stages for a particular lead easier. I'm not very confident about it though. I realize I'd be giving up on things like constraints, etc.

Are there any downsides to an approach like this?